1. Field of the Invention
The present invention relates to a power semiconductor module, and more specifically a power semiconductor module where a silicone gel provides insulating protection of a surface of an insulating substrate mounted with semiconductor chips.
2. Description of the Related Art
In recent years, power semiconductor modules are widely used for power converters with Insulated Gate Bipolar Transistors (IGBTs) to main semiconductor elements. A power semiconductor module has a structure where a single or a plurality of power semiconductor chips are incorporated in a resin case, and constitutes apart or the whole of power conversion and connection.
In manufacturing power semiconductor modules generally, power semiconductor chips, such as IGBT chips or diode chips, are mounted on an insulating substrate composed of ceramic, and these power semiconductor chips and insulating substrate are housed within a resin case, and further the insulating substrate and a metal base are solder-bonded to each other. The power semiconductor chips are electrically insulated from the metal base. The resin case is bonded and fixed with an adhesive to a peripheral edge of the metal base, and a silicone gel of low elasticity is filled up within the resin case in order to insulate and protect the surface of the insulating substrate and the power semiconductor chips on the insulating substrate. The resin case is composed of thermo-plastics, such as Polyphenylenesulfide (PPS). A cover composed of the same resin as the resin case is set over the resin case.
The following are disclosed with respect to the structure of a power semiconductor module. Japanese Unexamined Utility Model Application Publication No. H4-109550 (“Patent Document 1”) discloses a semiconductor device to cover semiconductor elements and wires with silicone gel, and further to fill silicone rubber foam between a power circuit board and a control circuit board. In addition, Japanese Unexamined Patent Application Publication No. H7-58282 (“Patent Document 2”) discloses a power semiconductor module wherein a coating of a rubberlike synthetic resin is formed on the inner face of a bonding part of resin case to metal base. Further, Japanese Unexamined Patent Application Publication No. 2005-57875 (“Patent Document 3”) discloses an inverter device with a diffusion preventive resin layer for preventing epoxy resin composition component transmission provided between an epoxy resin layer and a silicone gel layer sealing power semiconductor elements.
However, every structure of power semiconductor modules described in patent documents 1 to 3 insufficiently suppresses an influence of mutual reaction between silicone gel and component members forming the power semiconductor module, so that the suppression of silicone gel degradation can stand further improvement.
In recent years, an IGBT power semiconductor module has become required to perform continuous operation at high temperatures. A guarantee of continuous operation at high temperatures requires selection of one of accelerated life test conditions described in Table 3.1 of UL1557 for example, based on a guarantee temperature, to test the IGBT power semiconductor module under the selected condition, and then to enable maintenance of insulation breakdown resistance of the product standard as a pass criterion for the test.
Hardening degradation of silicone gel at high temperatures is cited as a degradation factor of insulation breakdown resistance in the test concerned. When hardening degradation of silicone gel makes progress, a crack of silicone gel is generated from a stress concentration spot in the inside of the IGBT power semiconductor module, and when the crack reaches the surface of the insulating substrate, preservation of insulation breakdown resistance becomes difficult.
In order to suppress the hardening degradation of silicone gel as mentioned above, there is a method of enhancing heat resistance of silicone gel by adding heat-resistant additives to silicone gel. However, even though silicone gel alone has sufficient heat resistance, at high temperatures of 200° C. or more, the hardening degradation is accelerated under the influence of component members of IGBT power semiconductor modules, and consequently inherent heat resistance of silicone gel alone may not be revealed. This phenomenon noticeably appears in silicone gel having a polydimethylsiloxane structure, most commonly known as a molecular structure of base polymer of silicone gel.
According to the above reason, even though evaluation of silicone gel alone results in sufficient heat resistance, in evaluating silicone gel applied to power semiconductor module products under UL1557, it may happen that insulation breakdown resistance cannot be preserved within the limits of the product standard after conducting the test under accelerated life test conditions.
One approach to a solution for this problem is to make a change into silicone gel, less influenced by mutual reaction with component members of power semiconductor modules (for example, of which the base polymer itself is provided with heat resistance by introducing high heat-resistant functional group into side chains of polydimethylsiloxane structure of the base polymer). By changing the base polymer, however, the property of the silicone gel greatly changes, and therefore it is possible that an unintended disadvantage happens where previously there was not a problem (for example, an increase of gel foam). In addition, the material cost of silicone gel very possibly increases, and further the time and cost required to research and develop new materials of silicone gel become necessary, and consequently the above-mentioned approach is not easily acceptable.
It is an object of the present invention to favorably solve the above-mentioned problems and to provide power semiconductor modules which allow the influence of mutual reaction between silicone gel and component members of power semiconductor modules to be suppressed and enable degradation of silicone gel to be suppressed thereby.